Heatbed will not heat; I've replaced the bed, the cables, AND the motherboard, still won't work.  

Heatbed will not heat; I've replaced the bed, the cables, AND the motherboard, still won't work.

I'm at a total loss as to what the problem can be. Several weeks ago, the bed just stopped heating. The thermistor is fine, the bed itself just will not heat. I replaced the bed, still won't heat. I replaced the cables connecting the motherboard to the heat bed, still won't heat. I bought a brand new motherboard and hooked everything up, STILL won't heat. The same thing constantly happens; I hit preheat, the LED on the bed briefly blinks green once, then nothing. 

I'm not sure what else can be done. I've attached a few pictures of the motherboard and bed set up, but frankly after talking to 3+ printing experts who are all equally baffled as to what the problem could be, I'm pretty sure I just have a completely dead printer.